Add support for devstack to install using system-scopes

[Lance Bragstad]

Right now devstack primarily runs under the assumption that project-admins can access deployment-level APIs (e.g., creating endpoints and services in keystone, creating compute hypervisors, creating public images, etc.)

This functionality is going to break when services start updating policies to use system-scope and removing the old functionality for project-administrators.

We should prepare devstack for that change by updating the installation process to use system-scope where applicable.

This topic was discussed during the Xena PTG [0].

[0] https://etherpad.opendev.org/p/policy-popup-xena-ptg

[Dr. Jens Harbott (j-harbott)]

For this I'd also prefer to have the installation documentation updated accordingly first, but I could understand if you would want to see things tested before doing so. Would you expect to just switch the process to using system scope or would you expect to need to be able to run both variants at least for some transition period?

[Dr. Jens Harbott (j-harbott)]

I think that this has mostly been completed (for projects that support it), there is even a job that tests it.